(1) Field of the Invention
The present invention relates to a process for the preparation of a grain oriented electrical steel sheet to be used for an iron core of an electric appliance. More particularly, the present invention relates to a process for the preparation of a thin steel sheet having improved iron loss characteristics.
(2) Description of the Related Art
A grain oriented electrical steel sheet is mainly used as a magnetic core material of a transformer or other electric appliance, and this grain oriented electrical material must have superior magnetic characteristics such as exciting characteristics and iron loss characteristics.
To obtain a steel sheet having superior magnetic characteristics, the &lt;001&gt; axis, which is the easy magnetization axis, must be highly oriented in the rolling direction. Furthermore, the magnetic characteristics are greatly influenced by the sheet thickness, the crystal grain size, the inherent resistance, and the surface film.
The orientation of an electrical steel sheet is greatly improved by the heavy reduction one-stage cold rolling process in which AlN or MnS is caused to function as an inhibitor, and currently, an electrical steel sheet having a flux density corresponding to about 96% of the theoretical value is used.
To cope with increasing energy costs, makers of transformers have an urgent need for magnetic materials having a reduced iron loss, as materials for energy-saving transformers.
High-Si materials such as amorphous alloys and 6.5% Si alloys have been developed as materials having a low iron loss, but the price and processability of these materials as the material for a transformer are unsatisfactory.
The iron loss of an electrical steel sheet is greatly influenced by not only the Si content but also the sheet thickness, and it is known that, if the thickness of the sheet is reduced by chemical polishing, the iron loss is reduced.
As the conventional process for preparing a thin grain oriented electrical steel sheet having a high flux density, the techniques disclosed in Japanese Unexamined Patent Publication No. 57-41326, Japanese Unexamined Patent Publication No. 58-217630, Japanese Unexamined Patent Publication No. 60-59044, Japanese Unexamined Patent Publication No. 61-79721, and Japanese Unexamined Patent Publication No. 61-117215, are known.
Japanese Unexamined Patent Publication No. 57-41326 discloses a preparation process in which a material comprising, as the inhibitor, 0.010 to 0.035% of at least one member selected from S and Se and 0.010 to 0.080% of at least one member selected from Sb, As, Bi and Sn is used as the starting material.
Japanese Unexamined Patent Publication No. 58-217630 discloses a preparation process in which a material comprising 0.02 to 0.12% of C, 2.5 to 4.0% of Si, 0.03 to 0.15% of Mn, 0.01 to 0.05% of S, 0.01 to 0.05% of Al, 0.004 to 0.012% of N and 0.03 to 0.3% of Sn or a material further comprising 0.02 to 0.3% or Cu is used as the starting material.
Japanese Unexamined Patent Publication No. 60-59044 discloses a preparation process in which a silicon steel material comprising 0.02 to 0.10% of C, 2.5 to 4.5% of Si, 0.04 to 0.4% of Sn, 0.015 to 0.040% of acid-soluble Al, 0.0040 to 0.0100% of N, 0.030 to 0.150% of Mn and 0.015 to 0.040% of S as indispensable components, and further comprising up to 0.04% of Se and up to 0.4% of at least one member selected from Sb, Cu, As, and Bi is used as the starting material.
Japanese Unexamined Patent Publication No. 61-79721 discloses a preparation process in which a silicon steel material comprising 3.1 to 4.5% of Si, 0.003 to 0.1% of Mo, 0.005 to 0.06% of acid-soluble Al and 0.005 to 0.1% of at least one member selected from S and Se is used as the starting material.
Japanese Unexamined Patent Publication No. 61-117215 discloses a preparation process in which a silicon steel material comprising 0.03 to 0.10% of C, 2.5 to 4.0% of Si, 0.02 to 0.2% of Mn, 0.01 to 0.04% of S, 0.015 to 0.040% of acid-soluble Al and 0.0040 to 0.0100% of N and further comprising up to 0.04% of Se and up to 0.4% of at least one member selected from Sn, Sb, As, Bi, Cu and Cr is used as the starting material.
All of the above prior art processes are based on the assumption that a hot rolling step is accompanied by heating a slab at a high temperature to control an inhibitor.